Variable speed oil-injected screw compressors

ABSTRACT

The invention relates to the use of variable speed control of the cooling fan on a variable speed oil-injected screw compressor. A screw compressor ( 11 ) comprises at least one stage of compression, each with a pair of rotors, variable speed compressor drive means ( 12 ), an oil reclaimer ( 13 ) from the compressed air and cooling apparatus ( 17 ) for cooling the oil extracted from the compressed air. The cooling apparatus comprises a heat exchange device and a fan ( 18 ) driven by a motor ( 19 ) which can be run at different speeds.

[0001] The invention relates to the use of variable speed control of thecooling fan on a variable speed oil-injected screw compressor.

[0002] An oil-injected screw compressor comprising one or more stages ofcompression can be driven from a variable speed motor. The speed of themotor is controlled automatically to drive the compressor either at oneof a series of pre-set speeds or to continuously adjust the speed sothat the output volume of the compressor matches the demand.

[0003] The oil that is used to cool, lubricate and seal the compressorelement is cooled in a radiator that uses ambient air as the coolingmedium. A fan is used to pass air over the radiator. The term “oil” asused in this specifically also applies to, and is intended to cover,synthetic oils or other similar coolants.

[0004] Conventionally the fan is driven by a fixed speed electric motor,which runs continuously whilst the compressor is running. Athermostatically controlled by-pass valve is generally used as a meansof diverting the oil away from the radiator until the oil temperaturereaches a certain value. The valve currently used is operated by aself-contained wax capsule. As the oil temperature increases, the waxexpands and operates the valve to divert the oil through the radiator.

[0005] In a variable speed compressor the quantity of heat rejected tothe cooling oil varies with the speed and pressure at which thecompressor is running. As the speed or pressure is reduced, less poweris required and therefore less heat is rejected to the oil. Whilstrunning under light load, or in cool conditions, there is a tendency forthe oil to overcool causing moisture in the compressed air to condense.Over a period of time, this water accumulates in the oil system. If thisis not regularly drained, water will circulate with the oil causingdamage to bearings and corroding ferrous surfaces.

[0006] It is therefore an object of the present invention to overcomethese disadvantages.

[0007] The invention therefore comprises a screw compressor comprisingat least one stage of compression, each compressor stage comprising apair of rotors, variable speed compressor drive means for driving atleast one of said rotors to effect air compression, an oil reclaimer forextracting oil from the compressed air, cooling apparatus for coolingoil extracted from the compressed air, wherein said cooling apparatuscomprises a heat exchange device and a fan driven by a motor which canbe run at different speeds to provide cooling air to the heat exchangedevice, said motor being independent from said variable speed compressordrive means, the speed of the fan motor being controlled by a controlunit, the control unit comprising processing means for processingsignals generated by a plurality of devices monitoring operatingparameters of the compressor, at least one of which monitoring devicesmonitors the speed of the variable speed compressor drive means, saidprocessing means calculating the input power of the compressor using acombination of the speed measurement and torque of the variable speedcompressor drive means, and adjusting the speed of the fanproportionally to the input power to balance the heat rejected to thecooling air with heat rejection to the oil.

[0008] A preferred embodiment of the present invention will now bedescribed, by way of example only, with reference to the accompanyingdrawing in which:

[0009]FIG. 1 is a schematic representation of a screw compressoraccording to the present invention.

[0010] Typically each compressor stage of a screw compressor 5 consistsof a pair of helically fluted rotors supported at each end in rollingbearings. The following description covers the operation of a singlestage compressor, but the invention applies in a similar manner tomulti-stage machines. A variable speed motor 12 drives one rotor, whichtransmits the drive to the counter-rotating rotor. The variable speedmotor 12 is used to drive the compressor 5 directly. An electroniccontrol system continuously adjusts the speed of the compressors, withinpre-set limits, so that the output flow matches the consumers demand tomaintain the designated system pressure.

[0011] Air is drawn through an air filter 10 into the compressor by theaction of the rotors. In the compression element 11, the air iscompressed between the rotors and the casing. During this process oil,at a higher pressure than that of the air, is injected into the airthrough a port in the compressor casing. The oil cools, lubricates andseals the compressor element 11. The oil/air mixture is furthercompressed and is then discharged through a delivery port of thecompression element 11 into an oil reclaimer 13. The oil is separatedfrom the air in the reclaimer 13. The separated oil from the reclaimer13 is then returned to the compression element 11 through an oil cooler,and an oil filter 20. The difference in air pressure between thereclaimer 13 and the injection point in the compression element 11drives the oil through this circuit. The compressed air leaves thereclaimer 13 through a fine filter 14, a non-return valve 15 and, inmost cases, an after-cooler 16.

[0012] The oil cooler comprises a radiator 17 and fan 18, which isdriven by a motor and control unit 19.

[0013] The cooling oil is itself cooled in the radiator 17, which usesambient air as a cooling medium. A fan 18 is used to pass a flow of airover the radiator 17.

[0014] A motor and control unit 19, drives the fan 18, which can becontrolled to run automatically either at a number of different fixedspeeds or the speed can be continuously varied, in response to controlsignals derived from certain parameters of the operating conditions ofthe compressor 5. This may be achieved by the use of one of thefollowing alternatives:

[0015] i) a pole change (or similar) motor that can be switched to runat two or more speeds;

[0016] ii) an induction motor that can be run via an electronic drive ata number of pre-determined speeds or varied continuously in response toa control signal;

[0017] iii) a switched reluctance drive motor that can be run via anelectronic drive at a number of pre-determined speeds or variedcontinuously in response to a control signal; or

[0018] iv) any other form of electric variable speed drive.

[0019] The operating parameters of the compressor 5 are continuouslymonitored by any appropriate monitoring devices. These parametersinclude speed and torque of the motor of the compressor drive, the airpressure of the air delivery point of the compressor 5 or at thedischarge point of the compressor 5, oil temperature at the oil cooleroutlet, ambient temperature and air/oil delivery temperature of thecompressor stages, in particular the final stage. Signals are generatedby the monitoring devices, which are fed to the electronic controller ofthe motor and control unit 19 and are processed to enable the controllerto adjust the fan speed to modify the heat energy being rejected fromthe oil to the cooling air. Essentially, by measuring the torque andspeed of the compressor drive, the input power to the compressor 5 canbe calculated, and the speed of the motor driving the fan 18 is adjustedproportionally to the input power, so that the heat rejected to thecooling air balances the heat rejection to the oil. The input powercould, alternatively be measured electrically using a kilowatttransducer.

[0020] The oil temperature at the oil cooler outlet, or other measuredparameters, is used to further adjust the speed of the fan 18 tocompensate for variations in ambient temperature, cooler efficiency andfan performance.

[0021] A variable speed drive used on a compressor 5 of this type offerssignificant efficiency improvements under part load conditions. This isbecause matching the output of the compressor 5 to the demand bycontrolling the speed is more efficient than other means of capacitycontrol.

[0022] However to have the cooling fan 18 running at full speed (andpower) irrespective of the compressor load reduces the overallefficiency improvement. A variable speed fan will mean that the fan 18only consumes the amount of energy necessary to cool the compressor oil.

[0023] A secondary benefit is that reducing the speed of the fan 18 willreduce the noise level of the compressor 5.

1. A screw compressor(s) comprising at least one stage of compression(11), each compressor stage comprising a pair of rotors, variable speedcompressor drive means (12) for driving at least one of said rotors toeffect air compression, an oil reclaimer (13) for extracting oil fromthe compressed air, cooling apparatus for cooling oil extracted from thecompressed air, wherein said cooling apparatus comprises a heat exchangedevice and a fan (18) driven by a motor (19) which can be run atdifferent speeds to provide cooling air to the heat exchange device,said motor (19) being independent from said variable speed compressordrive means (12), the speed of the fan motor (19) being controlled by acontrol unit (19), the control unit (19) comprising processing means forprocessing signals generated by a plurality of devices monitoringoperating parameters of the compressor (5), at least one of whichmonitoring devices monitors the speed of the variable speed compressordrive means (12), said processing means calculating the input power ofthe compressor (5) using a combination of the speed measurement andtorque of the variable speed compressor drive means, and adjusting thespeed of the fan (18) proportionally to the input power to balance theheat rejected to the cooling air with heat rejection to the oil.
 2. Ascrew compressor (5) as claimed in claim 1 in which the fan motor (19)can be run at a number of different fixed speeds, or the motor speed canbe continuously varied.
 3. A screw compressor as claimed in any one ofthe preceding claims in which at least one monitoring device monitorsthe torque of the variable speed compressor drive means.
 4. A screwcompressor (5) as claimed in any one of the preceding claims in which atleast one monitoring device monitors the torque of the variable speedcompressor drive means (5).
 5. A screw compressor (5) as claimed in anyone of the preceding claims in which at least one monitoring devicemonitors the pressure of the air at the compressor outlet.
 6. A screwcompressor (5) as claimed in any one of the preceding claims in which atleast one monitoring device monitors the temperature of the oil at anoutlet of the cooling apparatus.
 7. A screw compressor (5) as claimed inany one of the preceding claims in which at least one monitoring devicemonitors the ambient temperature.
 8. A screw compressor (5) as claimedin any one of the preceding claims in which at least one monitoringdevice monitors the air/oil delivery temperature of the finalcompression stage.